1. Field of the Invention
The present invention relates to an active matrix board, more particularly, it relates to an active matrix printed circuit board which is suitably applied to a liquid crystal display device such as a flat panel display for OA systems or TV set.
2. Description of the Related Art
A liquid crystal display panel having a large display area is strongly desired to be used for terminal equipments of OA devices or a liquid crystal TV set. Such a large sized liquid crystal display device is arranged to be provided with an active matrix printed circuit board to maintain the voltage applied thereto. Such an active matrix board comprises a substrate, pixel electrodes formed on the substrate, scanning electrodes also formed on the substrate, and a switch element formed between the pixel electrode and the scanning electrode for each pixel.
An MIM element is usually arranged as such a switch element of the active matrix board since the MIM element comprises a thin film two-terminal element which has a non-linear current-voltage characteristic that is desirable for switching function.
The thin film two-terminal element comprises an electrically insulating substrate such as a glass plate, a lower electrode made from a metal such as Ta, Al, Ti, etc., formed on the substrate, an electrically insulating film made from an oxide of the metal or SiOx or SiNx formed on the electrode, and an upper electrode made from a metal such as Al or Cr, etc., formed on the insulating film.
Examples of the thin film two-terminal element using a metal oxide as the insulator (insulating film) are disclosed in Japanese Patent Application Laying Open Nos. 57-196589, 61-232689 and 62-62333.
However, in accordance with the examples disclosed in the patent documents, the insulating film is formed by a thermal oxidizing process or an anodic oxidation process to oxidize the lower electrode. Therefore, the process for producing the element becomes complicated and necessitates a high temperature thermal treatment step (even in the anodic oxidation process, a thermal treatment step is conducted to remove impurities). Also, the film is not well controlled, that is, the film quality and film thickness are not constant and the reproductivity thereof is not satisfactory. Besides, the freedom of design is narrowed since the device material and the characteristic thereof are limited, because the substrate has to be made from a heat resisting material and the insulating film has to be made from a metal oxide that has constant physical properties.
Therefore, it is difficult to obtain a device that fully satisfies the requirements needed for one to be applied to the liquid crystal display device having the thin film two-terminal element.
Also, due to the insufficient film controllability, it is difficult to obtain a constant and stable element characteristic with regard to the current characteristic (I), the voltage characteristic (V), especially the I-V characteristic and the symmetricity of the I-V characteristic, that is, the current ratio I.sub.- /I.sub.+ between the current of the positive bias voltage and that of the negative bias voltage.
Further, when the thin film two-terminal element is applied to the liquid crystal display device, it is desirable in general that the ratio of the capacity of the liquid crystal portion with respect to that of the thin film two-terminal element be more than 10. However, the capacity of the known thin film two-terminal element is large since the metal oxide film has a large dielectric constant. Therefore, it becomes necessary to reduce the size of the element to decrease the capacity thereof, which necessitates a fine processing of the element. This also results in that the through-put of the production is lowered since the insulating film is mechanically damaged at the time of rubbing process when the liquid crystal material is to be sealed in the device.
Besides, there is an appropriate range of element characteristic for driving the liquid crystal at a low duty ratio, more precisely below 1/400 wherein the active matrix arrangement is to be provided to drive the liquid crystal. When a plurality of elements are arranged within the display surface and the elements are to be driven at the same condition, the characteristic dispersion of the elements in the display surface has to be within the range mentioned above. However, such a characteristic range has not been prescribed so far, so that the dispersion of the characteristic between the elements becomes large, which results in the unevenness of display.